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Experiment 4: Probability of selecting the target as a function of relative contrast of target or distractor. Data from individual observers. Filled symbols: the contrast of the target is varied. Open symbols: the contrast of the distractor is varied. The line is the ML best fit. Individual thresholds are given by contrast values corresponding to 50% level performance and are, respectively, 0.65 ± 0.01, 0.62 ± 0.006, 0.62 ± 0.01, 0.61 ± 0.01, 0.67 ± 0.01.
